1. Field of the Disclosure
Exemplary embodiments of the present disclosure relate to a fixing device and an image forming apparatus including the fixing device, and more specifically, to a fixing device that applies heat and pressure to a recording medium at a nip between a fixing belt and a pressing member to fix an image on the recording medium and an image forming apparatus including the fixing device.
2. Description of the Background
As one type of image forming apparatus, electrophotographic image forming apparatuses are widely known. In an image formation process executed by an electrophotographic image forming apparatus, for example, a charger uniformly charges a surface of an image carrier (e.g., photoconductive drum); an optical writing unit emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
FIG. 1 is a schematic configuration view illustrating a conventional belt-type fixing device. In FIG. 1, the belt-type fixing device includes a heating roller 202, a fixing roller 203, a fixing belt 204, and a pressing roller 205. The heating roller 202 includes a heater 201. The fixing roller 203 includes a rubber layer on its surface. The fixing belt 204 is stretched between the heating roller 202 and the fixing roller 203. The pressing roller 205 presses against the fixing roller 203 via the fixing belt 204 to form a fixing nip N.
When a toner image is transferred onto a recording medium P, the recording medium P is conveyed to the fixing nip N between the fixing belt 204 and the pressing roller 205. When the recording medium P passes the fixing nip N, heat and pressure are applied to the toner image on the recording medium P to fix the toner image.
FIG. 2 is a schematic configuration view illustrating a conventional film-type fixing device. As described in JP-H04-044075-A, typically, a ceramic heater 211 and a pressing roller 212 sandwich a heat-resistant film (fixing belt) 213 to form the fixing nip N.
A recording sheet is fed to the fixing nip N between the heat-resistant film 213 and the pressing roller 212. Then, the recording sheet is sandwiched by the heat-resistant film 213 and the pressing roller 212 to be conveyed together with the heat-resistant film 213.
The film-type fixing device may be an on-demand type fixing device including a ceramic heater and a film member of low heat capacity. Further, in an image forming apparatus including the fixing device, only during image formation, the ceramic heater may be turned on to generate heat at a certain fixing temperature to shorten a waiting time required from turning-on of the image forming apparatus to a state ready for image formation and reduce the power consumption at a standby mode.
Finally, a conventional pressing-belt-type fixing device like that described in JP-H08-262903-A includes a heat fixing roller, an endless belt, and a pressing pad. The heat fixing roller is rotatable and has an elastically deformable surface. The endless belt travels in contact with the heat fixing roller. The pressing pad is fixedly mounted inside a loop formed by the endless belt and presses the endless belt against the heat fixing roller to form a belt nip between the endless belt and the heat fixing roller through which the recording medium passes.
According to the pressing-belt-type fixing device described above, pressure of the pressing pad elastically deforms the surface of the heat fixing roller and extends a contact area of the heat fixing roller and the recording medium to enhance heat conduction efficiency, reduce energy consumption, and achieve downsizing.
However, for example, in the above-described film-type fixing device described in JP-H04-044075-A, there is room for improvement in durability and temperature stability of the fixing belt.
For example, the abrasion resistance of a sliding surface between the ceramic heater and the fixing belt made of heat-resistant film may be insufficient. When the fixing belt is driven for an extended period of time, the sliding surface is worn out. Accordingly, traveling of the fixing belt may become unsteady and/or the driving torque of the fixing device may increase. Consequently, the recording medium may slip on the fixing belt, causing displacement of a resultant image. Alternatively, increased stress may be applied to a driving gear, causing damage to the gear.
Further, in the film-type fixing device, the fixing belt is partially heated at the fixing nip. The temperature of the fixing belt is at its lowest when the fixing belt in rotation returns to an entrance of the fixing nip, causing faulty fixing particularly at high-speed rotation.
To reduce the friction between the fixing belt and the ceramic heater or other stationary member, for example, JP-H08-262903-A describes the fixing device using a fiberglass sheet impregnated with polytetrafluoroethylene (PTFE) as a low-friction sheet (a sheet-shaped slide member) on a surface layer of the pressing pad.
However, in the above-described pressing-belt-type fixing device, a large heat capacity of the fixing roller may increase the time required for raising the temperature of the fixing roller, thereby extending the warm-up time.
To deal with such challenges, JP-2007-334205-A proposes a fixing device including an endless fixing belt and a heat conductive member of metal fixed in a loop formed by the endless fixing belt. In the fixing device, lubricant is provided between the endless fixing belt and the heat conductive member to reduce the friction resistance caused when the endless belt slides against the heat conductive member.
In the above-described fixing device of film-heating or pressing-belt type, such a sliding portion between the endless belt and the heat conductive member is limited to the fixing nip or a nearby portion thereof. On the other hand, in the fixing device described in JP-2007-334205-A, the endless belt slides over a substantially entire circumference of the heat conductive member. Thus, lubricant needs to cover the substantially entire circumference of the sliding portion.
However, in the configuration described in JP-2007-334205-A, since the viscosity of lubricant is high at low temperatures, the friction resistance of the sliding portion is increased, causing an increased torque applied to a motor serving as a driving unit. Consequently, stability in the rotation speed of the motor might be reduced.